Need help with a filter leak problem

I have an older in-ground Blue Haven pool. The pool was built in about 1992 and is in very good condition. We have lived at this house since 2004 and with the help of this forum have saved a ton of money. My most recent problem has me baffled and feeling like a complete beginner again. The problem is a water leak from my filter. The filter is a Hayward Starclear II C1100 single cartridge filter which I believe to be as old as the pool. I replaced the pump last year and keep the cartridge clean and it all seems to work very well. Recently it began to leak from the o-ring between the filter body and the filter head. The o-ring was replaced but the leak continued. At that point I began to check everything closer and discovered some hairline cracks in the filter head. I had read they could be fixed with epoxy or JB weld but decided to purchase a new filter head. I checked the filter body and found no obvious defects and another new o-ring was used with the new filter head. When started with the new parts it still leaks from the joint. The filter body seems level and there are no visible defects. The water seems to be pushing out from around the o-ring. The gauge in the new filter head indicates the pressure in the system is where it has always been and there is nothing else that has been changed. I have tried it with o-ring lube, different tension on the locking nut and anything else I can think of but this leak wont stop. When I close the system it does not leak initially but as soon as the air purges from the system through the returns the leak comes back. Has anybody experienced this before and if so, what fixed it. I would hate to have to purchase the entire filter system because I cant get the o-ring to seal.

Thank you in advance...

Hopefully someone will spot it who can help you. I'm not familiar enough with carts to help.

I've got a different Hayward cartridge filter. It sounds like they're similar. I would make sure you bought the correct o ring. It could be that the diameter of the rubber on the one your purchased is too small. Also, I've found that greasing the o rings tends to cause more problems than it solves. Try cleaning the o ring off, and scrubbing any gunk or lubricant that might be inside the ridge that the ring sits in.

Also, visually inspect the locking ring that connects the two halves of the filter. If it's damaged, you might not be getting a good seal because of the damage.

What kelemvor said. Check that the threaded rod is all the way down in the lower body. If not, your top nut may not have enough threads to tighten all the way. Run the top nut down on the rod without the top housing in place to be sure it threads easily. Did you replace the cartridge? Some look identical, but have slightly different dimensions. (Too tall?) Is the cartridge all the way to the bottom, etc. Take it out and try it.

Not having a leak while the air purges I think would be expected, since there's no pressure to speak of yet.